This invention relates in general to apparatus for cutting sheet material and deals more particularly with improved apparatus for cutting pattern pieces from a single sheet or a layup comprising relatively few sheets of material and having a cutter which moves in cutting engagement with a bearing surface upon which sheet material to be cut is spread.
In an apparatus of the aforedescribed general type, the sheet material to be cut must be retained in firmly fixed position on the bearing surface so that it cannot shift during the cutting operation. Heretofore, apparatus of the aforedescribed type has been provided wherein sheet material to be cut is adhered to a bearing surface. One such apparatus utilizes a tacky coating on its bearing surface which cooperates with sheet material to releasably hold it in relatively firmly fixed position. While such apparatus has proven generally satisfactory, a new coating of tacky material must be periodically applied to the supporting or bearing surface to maintain the apparatus in proper working condition. This resurfacing operation necessitates costly machine downtime. Further, this arrangement is not particularly satisfactory for holding a layup which comprises a plurality of stacked plies, because the tacky material engages and holds only the lowermost ply of the layup. The sheets or plies thereabove may be prone to shifting movement when engaged by the cutter despite frictional engagement between adjacent plies.
Electrostatic holddown apparatus has also been used successfully to secure a single ply or layup comprising a relatively few plies of material in fixed position on a bearing surface, however, considerably auxiliary equipment is required for generating a force field which makes such apparatus costly to produce.
Vacuum holddown devices of the type which produce subatmospheric pressure at a work or bearing surface on which sheet material is spread have gained widespread acceptance in the material cutting art. A vacuum holddown device of the latter type is illustrated and described in U.S. Pat. No. 3,772,949 to Pavone et al for Method and Apparatus for Cutting Sheet Material, issued Nov. 20, 1973. The apparatus illustrated and described in the aforesaid patent includes a cutting wheel which rolls in cutting engagement with a bearing surface which has an array of openings therethrough communicating through channels with a vacuum pump. The patented apparatus is particularly adapted to cut anisotropic sheet materials, such as fibrous composite tapes which have different cutting characteristics along different principle axes. However, problems may be encountered when such apparatus is used to cut other types of materials, as, for example, woven fabrics. As the cutting instrument passes over holes in the bearing surface, threads which comprise the fabric may be forced into the holes by the cutting instrument rather than being cut by it. As a result, pattern pieces cut from the fabric are not easily separated from the waste material produced by the cutting operation. Further, failure to effect clean shearing of all of the threads which comprise the fabric may result in cut pattern pieces with rough or ragged edges. The present invention is primarily concerned with the aforesaid problems.